This paper studies the outage performance behaviour of underlay cooperative cognitive networks over Nakagami-m fading channels. For time savings in collecting simulation results, the authors propose a new outage probability expression. Notably, this expression is quite general, taking into account practical operation conditions such as interference power constraint, maximum transmit power constraint, correlation among received signal-to-noise ratios, non-identically and independently distributed Nakagami-m fading channels and the maximum ratio combining receiver. Moreover, it is useful in obtaining numerical results for performance comparison between dual-hop communications and cooperative communications in underlay cognitive networks. Interestingly, analytical results illustrate that like dual-hop cognitive networks, cooperative cognitive ones experience an error floor but the latter significantly outperform the former at almost no cost of power and bandwidth.